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    <meta property="og:title" content="Program Annotations" />
<meta property="og:description" content="In Pony, we provide a special syntax for implementation-specific annotations to various elements of a program. The basic syntax is a comma-separated list of identifiers surrounded by backslashes:
\annotation1, annotation2\ Here, annotation1 and annotation2 can be any valid Pony identifier, i.e. a sequence of alphanumeric characters starting with a letter or an underscore.
What can be annotated Annotations are allowed after any scoping keyword or symbol. The full list is:" />
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<meta property="article:modified_time" content="2020-04-11T18:34:49+08:00" />
<meta itemprop="name" content="Program Annotations">
<meta itemprop="description" content="In Pony, we provide a special syntax for implementation-specific annotations to various elements of a program. The basic syntax is a comma-separated list of identifiers surrounded by backslashes:
\annotation1, annotation2\ Here, annotation1 and annotation2 can be any valid Pony identifier, i.e. a sequence of alphanumeric characters starting with a letter or an underscore.
What can be annotated Annotations are allowed after any scoping keyword or symbol. The full list is:">
<meta itemprop="dateModified" content="2020-04-11T18:34:49&#43;08:00" />
<meta itemprop="wordCount" content="334">



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<meta name="twitter:title" content="Program Annotations"/>
<meta name="twitter:description" content="In Pony, we provide a special syntax for implementation-specific annotations to various elements of a program. The basic syntax is a comma-separated list of identifiers surrounded by backslashes:
\annotation1, annotation2\ Here, annotation1 and annotation2 can be any valid Pony identifier, i.e. a sequence of alphanumeric characters starting with a letter or an underscore.
What can be annotated Annotations are allowed after any scoping keyword or symbol. The full list is:"/>

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              字面量（Literals）
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              比较（Equality in Pony）
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              匿名对象（Object Literals）
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              引用权能（Reference Capabilities）
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              权能约束（Reference Capability Guarantees）
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              权能借用（Recovering Capabilities）
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              别名引用（Aliasing）
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              Passing and Sharing References
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              权能包含关系（Capability Subtyping）
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              权能合并（Combining Capabilities）
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              箭头的用法（Arrow Types aka Viewpoints）
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              权能矩阵（Reference Capability Matrix）
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              对象权能模型（Object Capabilities）
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              边界信任（Trust Boundary）
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              匹配表达式（Match Expressions）
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              as操作符（As Operator）
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              概述（Overview）
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              泛型和引用权能（Generics and Reference Capabilities）
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              泛型约束（Constraints）
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  <ul>
    <li><a href="#what-can-be-annotated">What can be annotated</a></li>
    <li><a href="#the-effect-of-annotations">The effect of annotations</a>
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              APPENDICES
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            <h1 class="f1 athelas mb1">Program Annotations</h1>
          </header>

          <main class="nested-copy-line-height lh-copy serif f4 nested-links nested-img mid-gray pr4-l"><p>In Pony, we provide a special syntax for implementation-specific annotations to various elements of a program. The basic syntax is a comma-separated list of identifiers surrounded by backslashes:</p>
<pre><code class="language-pony" data-lang="pony">\annotation1, annotation2\
</code></pre><p>Here, <code>annotation1</code> and <code>annotation2</code> can be any valid Pony identifier, i.e. a sequence of alphanumeric characters starting with a letter or an underscore.</p>
<h2 id="what-can-be-annotated">What can be annotated</h2>
<p>Annotations are allowed after any scoping keyword or symbol. The full list is:</p>
<ul>
<li><code>actor</code></li>
<li><code>class</code></li>
<li><code>struct</code></li>
<li><code>primitive</code></li>
<li><code>trait</code></li>
<li><code>interface</code></li>
<li><code>new</code></li>
<li><code>fun</code></li>
<li><code>be</code></li>
<li><code>if</code> (only as a condition, not as a guard)</li>
<li><code>ifdef</code></li>
<li><code>elseif</code></li>
<li><code>else</code></li>
<li><code>while</code></li>
<li><code>repeat</code></li>
<li><code>until</code></li>
<li><code>for</code></li>
<li><code>match</code></li>
<li><code>|</code> (only as a case in a <code>match</code> expression)</li>
<li><code>recover</code></li>
<li><code>object</code></li>
<li><code>{</code> (only as a lambda)</li>
<li><code>with</code></li>
<li><code>try</code></li>
<li><code>then</code> (only when part of a <code>try</code> block)</li>
</ul>
<h2 id="the-effect-of-annotations">The effect of annotations</h2>
<p>Annotations are entirely implementation-specific. In other words, the Pony compiler (or any other tool that processes Pony programs) is free to take any action for any annotation that it encounters, including not doing anything at all. Annotations starting with <code>ponyint</code> are reserved by the compiler for internal use and shouldn&rsquo;t be used by external tools.</p>
<h3 id="annotations-in-the-pony-compiler">Annotations in the Pony compiler</h3>
<p>The following annotations are recognised by the Pony compiler. Note that the Pony compiler will ignore annotations that it doesn&rsquo;t recognise, as well as the annotations described here if they&rsquo;re encountered in an unexpected place.</p>
<h4 id="packed"><code>packed</code></h4>
<p>Recognised on a a <code>struct</code> declaration. Removes padding in the associated <code>struct</code>, making it ABI-compatible with a packed C structure with compatible members (declared with the <code>__attribute__((packed))</code> extension or the <code>#pragma pack</code> preprocessor directive in many C compilers).</p>
<pre><code class="language-pony" data-lang="pony">struct \packed\ MyPackedStruct
  var x: U8
  var y: U32
</code></pre><h4 id="likely-and-unlikely"><code>likely</code> and <code>unlikely</code></h4>
<p>Recognised on a conditional expression (<code>if</code>, <code>while</code>, <code>until</code> and <code>|</code> (as a pattern matching case)). Gives optimisation hints to the compiler on the likelihood of a given conditional expression.</p>
<pre><code class="language-pony" data-lang="pony">if \likely\ cond then
  foo
end

while \unlikely\ cond then
  bar
end

repeat
  baz
until \likely\ cond end

match obj
| \likely\ expr =&gt; foo
| \unlikely\ let capt: T =&gt; bar
end
</code></pre></main>
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